The evaporation calculation

by Jeremy Reets

A train leaves Chicago for Indianapolis (approximately 185 miles) at 9 a.m. One hour later, a train leaves Indianapolis for Chicago. They meet at noon.If the second train had started at 9 a.m., and the first train at 10:30 a.m., they would still have met at noon.Can you find the speed of each train?There is enough information in the above problem to tell you the speed of the trains. You just have to know what information is important and what to do with it once you have it.Evaporation applicationWhen it comes to the speed of evaporation, the same is true: We have enough information, we just need to know what information is important and what to do with it.This is very important, because when you look at what we are hired to do as water damage restoration contractors (remove excess moisture), evaporation plays a major role.As the experts, we are expected to make educated decisions when it comes to removing excess moisture from a structure.That means that you should be able to provide an equipment plan that is based on a credible evaporation calculation.Don’t confuse this with the dehumidifier calculators that we are currently using. That calculator does not tell you how effective you have set up your drying system for evaporation; instead, it provides a guideline for how many dehumidifiers you might use to control the vapor you have due to evaporation.Moisture motivationThe force that motivates moisture is both defined and measured as vapor pressure.If you have high vapor pressure in one room and lower vapor pressure in the room next to it, which way will vapor move?It will move from higher vapor pressure to lower until there is an equalization of the pressure.The same thing is true of evaporation.Moisture always moves toward lower pressure, if it can. The greater the difference in the pressures indicates the speed moisture will move.Evaporation takes place when the vapor pressure of the liquid is greater than the vapor pressure of the air.The higher the vapor pressure in the water at the surface and the lower the vapor pressure in the air at that surface, the faster evaporation takes place.

This is nothing new. Henri Le Châtelier first put this principle into writing in 1884.Evaporation calculationCalculating evaporation potential (EP) is very easy. We just need to measure the vapor pressure of the liquid on or in the wet surface, and compare it to the vapor pressure of the air at the wet surface.If we were to write that out it would be: "Vapor Pressure of the Wet Surface(S) minus Vapor Pressure of the Air(A) at the Wet Surface equals Evaporation Potential(EP)."S – A = EPYou only need three pieces of information to calculate evaporation potential:1. Temperature of the air2. Relative humidity of the air3. Temperature of the wet surfaceYou will need to have an infrared thermometer in order to get the surface temperature.Without surface temperature, you cannot be sure whether you are creating evaporation.The math that you put those numbers through to get vapor pressure is pretty complicated but, fortunately, all the hard work has been done for you.The higher the EP number, the greater the potential is for structure evaporation.Why do we use the term "evaporation potential" and not "rate of evaporation"? There are some factors that cannot be accurately measured yet, like airflow.Airspeed will be very different in every location of the room that you take a reading, and you will not get an accurate answer.

Creating vapor pressure differentialAs you already know, vapor pressure as a measurement is another specific humidity reading.Other specific humidity readings include grains per pound and dew point.The only way to lower specific humidity is to dehumidify or remove vapor from the drying environment.On a surface that is saturated, we read the relative humidity as 100 percent.If you get a psychrometric chart, you will see that the only way to increase the vapor pressure of a wet surface is to heat that surface.You can create very high vapor pressure when you heat a wet surface.Heating air does nothing unless you are transferring that heat into the water. (See "Vapor pressure differential")Application of evaporation calculationOn your next job, take some readings, including wet surface temperature.Using the evaporation calculator to figure out the best way to set equipment is simple.Once you put in your baseline readings, you can project how your efforts will affect the environment.

· To what temperature will you potentially heat the surface? · To what can you possibly lower the specific humidity? · What is the EP of that equipment configuration? · How will heating the surface affect the EP? · How will lowering the specific humidity affect the EP?Take some time with the evaporation calculator and try many different scenarios.You might be amazed at what this tool opens your mind to.You will probably realize that drying knowledge has changed significantly in the last year.There is so much new information in our industry that this is a great time to get an education update. http://www.advantagecleaningteam.com/ or http://www.janiservu.com/